The present invention relates to a process for disposable work-up of metal chlorides which accrue as cyclone dust in preparation of titanium dioxide via the chloride process, and, more particularly, to a process for treating the metal chlorides through pasting up of the cyclone dust in water and/or an acid to dissolve the metal chlorides, separation of the inert components of the cyclone dust from the suspension in a first separation step, selective precipitation of the metals as metal hydroxides via a gradual increase of the pH in the filtrate of the suspension to a value below 12, separation of the precipitated metal hydroxides in a second separation step, and work up of the precipitated metal hydroxides into disposable filter cakes.
In the commercial manufacture of titanium tetrachloride, a titanium containing ore, e.g. ilmenite, slag, synthetic rutile or natural rutile is chlorinated in the presence of coke as a reducing agent in a fluid bed reactor at temperatures of around 1000.degree. C. as described in U.S. Pat. No. 4,961,911. Besides the desired reaction product, namely titanium tetrachloride, the impurities contained in the ore are also continuously chlorinated, with the formation of especially iron (II) chloride (but not iron (III) chloride). Besides titanium tetrachloride, the material leaving the reactor contains higher boiling metal chlorides, low boiling components, such as carbon monoxide, carbon dioxide, carbon oxysulfide, hydrogen chloride and inert solid materials, especially coke powder as well as silicon dioxide and titanium dioxide.
The high boiling chlorides are condensed by cooling and, together with the inert solids, are separated from the gas stream in a cyclone separator. The product accruing in the cyclone is called "cyclone dust" in the following description. The designation "residual" or "waste chloride" can also be used to describe the accrued product.
The cyclone dust is further treated with well known processes. One such process involves pasting up and filtration. The filtrate of the suspension (essentially a solution containing iron (II) chloride) is, for example, suitable for sludge conditioning in waste water treatment and is, therefore, a marketable product. The coke-containing filtration residue is disposable or is useful as, for example, a fuel. Another such known further treatment process involves pasting up, neutralization and filtration. All metal chlorides are converted into a solid, water-insoluble form as hydroxides and, together with the inert materials, a disposable, well filterable, non-thixotropic filter cake is obtained as discussed in U.S. Pat. No. 3,655,344). Yet another further treatment process involves dry neutralization and pelletizing as discussed, for example, in DE 27 36 622 A1. On account of today's very strict requirements for disposal of waste materials, however, such a process is not practical.
A process for selective work up of the metal chlorides is described in EP 390 293 A1, in which the metal chlorides brought into solution are separated in a first step from the inert constituents and then metal hydroxides are selectively precipitated from the separated solution through addition of basic medium in the pH range of 3.5-4, with subsequent separation and drying. This known process does not take into account, however, a serious problem has not been carefully considered, namely the precipitated and worked up metal hydroxides obtained from the suspension without inert materials are thixotropic and thus cannot be deposited in a garbage dump.
An object of the present invention is to overcome the problems associated with the known processes by finding a process for the work up of the cyclone dust in which the inert components are separated and the metal chlorides are brought into a form which permits disposal.
The foregoing object has been achieved by selective precipitation so that the pH of the filtrate of the suspension will go above 5.0 only after addition of an oxidizing agent in the pH range of 4.2 to 5.
A special advantage in using the process according to the present invention is that the inert constituents of the cyclone dust for production of a disposable product are not required so as to considerably reduce the quantity of accrued filter cake. This is a great advantage, not only in the confines of the dump area but, also because the inert materials can be introduced into other chemical processes as valuable materials. Moreover, the filter cake accruing in the process is not thixotropic and is, therefore, disposable.
The critical point in the process of the present invention appears to be a partial oxidation of the divalent iron with simultaneous "ripening" of the filtrate of the suspension under special conditions. Thereafter, by increasing the pH, all metal hydroxides are precipitated and converted into a form which permits large scale disposal. It further appears that, with waiting or during the oxidation, the hydroxides of aluminum, titanium, chromium, vanadium, zirconium and niobium, which are otherwise filterable with difficulty and all are already formed at a pH of .ltoreq.4.5, so ripen that later in the adjustment of the filtrate of the suspension to a pH range to 10, a large quantity, above all of iron hydroxide, is precipitated on the already formed hydroxides. As an apparent consequence, even without the inert components, coke, titanium dioxide and silicon dioxide, a well filterable, non-thixotropic solid material is obtained.
According to an advantageous processing method according to the present invention, the suspension is to be so diluted further that the iron content is under 20 g/l.
In the oxidation treatment in the pH range between 4 and 5, introduction of air is especially preferred. Per cubic meter of suspension of 20 Nm.sup.3, air is introduced in a time interval of 10 to 100 minutes as an order of magnitude.